Semiconductor element and production method for same

ABSTRACT

A semiconductor element includes a high-resistivity substrate that includes a β-Ga 2 O 3 -based single crystal including an acceptor impurity, an undoped β-Ga 2 O 3 -based single crystal layer formed on the high-resistivity substrate, and an n-type channel layer that includes a side surface surrounded by the undoped β-Ga 2 O 3 -based single crystal layer. The undoped β-Ga 2 O 3 -based single crystal layer includes an element isolation region.

TECHNICAL FIELD

The invention relates to a semiconductor element and a production methodfor the same, in particular, to a β-Ga₂O₃-based semiconductor elementand a production method for the same.

BACKGROUND ART

In conventional semiconductor elements, an element isolation structureis used to electrically isolate elements arranged on a semiconductorlaminate. This type of element isolation structure is formed by, e.g.,an element isolation process using ion implantation of acceptor impurity(see, e.g., PTL 1).

In the conventional semiconductor device described in PTL 1, a p⁺-typechannel stop layer for element isolation is formed in an elementisolation region on a surface of a p-type silicon substrate.

CITATION LIST Patent Literature [PTL 1]

JP-A-H11-97519

SUMMARY OF INVENTION Technical Problem

In the element isolation using the acceptor impurity ion implantation, ahigh dose of acceptor impurity ions is implanted from the upper surfaceof the element isolation region to a deep position to reach thesubstrate. Therefore, implantation time is long and this makesmanufacturing process longer, resulting in that production takes longtime and also it is difficult to reduce the production cost.

Therefore, it is an object of the invention to provide a semiconductorelement and a production method thereof which allow production steps tobe simplified and production costs to be reduced.

By the way, with regard to nitride-based semiconductors and oxide-basedsemiconductors such as β-Ga₂O₃ etc., it is assumed that undoped crystalsthereof become n-type. This is because the cleaning effect of rawmaterials and apparatuses is limited so that it is difficult tocompletely prevent the unintentional contamination of donor impurities.Also, another reason is that crystal defects such as holes mayfrequently function as a donor and it is difficult to completely removethe crystal defects.

The present inventors have keenly studied the undoped crystals, and havethereby found that β-Ga₂O₃-based single crystals can be easily grown bythe generally known crystal growth methods so as to be high-resistivityundoped crystals and unexpectedly that the above object can be attainedby using the undoped crystals for the element isolation so as tocomplete the present invention.

To achieve the above object, the invention provides a semiconductorelement defined by [1] to [12] below and a production method for asemiconductor element defined by [13] to [15] below.

[1] A semiconductor element, comprising: a high-resistivity substratethat comprises a β-Ga₂O₃-based single crystal including an acceptorimpurity; an undoped β-Ga₂O₃-based single crystal layer formed on thehigh-resistivity substrate; and an n-type channel layer that comprises aside surface surrounded by the undoped β-Ga₂O₃-based single crystallayer, wherein the undoped β-Ga₂O₃-based single crystal layer comprisesan element isolation region.

[2] A semiconductor element, comprising: a high-resistivity substratethat comprises a β-Ga₂O₃-based single crystal comprising an acceptorimpurity; an undoped β-Ga₂O₃-based single crystal layer formed on thehigh-resistivity substrate; and an n-type channel layer that comprises aside surface and a bottom surface on a side of the substrate that aresurrounded by the undoped β-Ga₂O₃-based single crystal layer, whereinthe undoped β-Ga₂O₃-based single crystal layer comprises an elementisolation region.

[3] The semiconductor element according to [1] or [2], wherein theundoped β-Ga₂O₃-based single crystal layer is a region that includes anunintentional donor and/or acceptor impurity at a concentration of lessthan 1×10¹⁵ cm⁻³.

[4] The semiconductor element according to [1] or [2], wherein aconcentration of a donor impurity doped into the n-type channel layer isset to be higher than a concentration of an acceptor impurity of theundoped β-Ga₂O₃-based single crystal layer.

[5] The semiconductor element according to [1] or [2], comprising aMESFET or MOSFET.

[6] The semiconductor element according to [1] or [2], comprising anundoped region between an n-type channel region and an n-type channelregion.

[7] The semiconductor element according to [1] or [2], wherein theundoped β-Ga₂O₃-based single crystal layer is located between thehigh-resistivity substrate and the n-type channel layer.

[8] A semiconductor element, comprising: a high-resistivity substratethat comprises a β-Ga₂O₃-based single crystal including an acceptorimpurity; a low-concentration acceptor impurity-including β-Ga₂O₃-basedsingle crystal layer formed on the high-resistivity substrate; and ann-type channel layer that comprises a side surface and a bottom surfaceon a side of the substrate that are surrounded by the low-concentrationacceptor impurity-including β-Ga₂O₃-based single crystal layer, whereinthe low-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises an element isolation region.

[9] The semiconductor element according to [8], wherein thelow-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises a region that includes the acceptor impuritydiffused from the high-resistivity substrate at a concentration of lessthan 1×10¹⁶ cm⁻³.

[10] The semiconductor element according to [8] or [9], wherein a donorconcentration of the low-concentration acceptor impurity-includingβ-Ga₂O₃-based single crystal layer is set to be lower than aconcentration of the acceptor impurity diffused from thehigh-resistivity substrate, and wherein a concentration of a donorimpurity doped into the n-type channel layer is higher than aconcentration of an acceptor impurity of the undoped β-Ga₂O₃-basedsingle crystal layer.

[11] The semiconductor element according to [8] or [9], wherein thelow-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises a region that includes an intentionally dopedacceptor impurity at a concentration of less than 1×10¹⁶ cm³.

[12] The semiconductor element according to [8], wherein the n-typechannel layer comprises a side surface and a bottom surface on a side ofthe substrate that are surrounded by the acceptor impurity-includingβ-Ga₂O₃-based single crystal layer of a same element and a sameconcentration.

[13] A production method for a semiconductor element, comprising: a stepof forming an undoped β-Ga₂O₃-based single crystal layer on ahigh-resistivity substrate that comprises a β-Ga₂O₃-based single crystalincluding an acceptor impurity; and a step of forming an n-type channellayer by doping a donor impurity into a predetermined region of theundoped β-Ga₂O₃-based single crystal layer such that a side surfacethereof is surrounded by the undoped β-Ga₂O₃-based single crystal layer,wherein the undoped β-Ga₂O₃-based single crystal layer comprises anelement isolation region.

[14] A production method for a semiconductor element, comprising: a stepof forming a low-concentration acceptor impurity-including β-Ga₂O₃-basedsingle crystal layer on a high-resistivity substrate that comprises aβ-Ga₂O₃-based single crystal including an acceptor impurity; and a stepof forming an n-type channel layer by doping a donor impurity into apredetermined region of the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer such that a sidesurface and a bottom surface on a side of the substrate are surroundedby the low-concentration acceptor impurity-including β-Ga₂O₃-basedsingle crystal layer, wherein the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer comprises anelement isolation region.

[15] The production method for a semiconductor element according to[14], wherein the step of forming the low-concentration acceptorimpurity-including a β-Ga₂O₃-based single crystal layer comprises a stepof doping an acceptor impurity of less than 1×10¹⁶ cm⁻³ into an undopedβ-Ga₂O₃-based single crystal layer to form the low-concentrationacceptor impurity-including β-Ga₂O₃-based single crystal layer.

In the present invention, the undoped β-Ga₂O₃-based single crystal layermeans a layer comprised of a β-Ga₂O₃-based single crystal that includesa donor impurity and/or acceptor impurity of less than 1×10¹⁵ cm³ whichare not intentionally doped, and the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer means a layercomprised of a β-Ga₂O₃-based single crystal that includes an acceptorimpurity of less than 1×10¹⁶ cm⁻³. As the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer, included are,e.g., a β-Ga₂O₃-based single crystal layer into which a trace amount ofacceptor impurity is doped for enhancing the safety against thecontamination of an unintentional donor impurity, and a β-Ga₂O₃-basedsingle crystal layer which includes a trace amount of acceptor impuritydiffused from a acceptor impurity doped layer (e.g., a high-resistivitysubstrate). Herein, the β-Ga₂O₃-based single crystal means a singlecrystal with a composition of β-(Ga_(x)In_(y)Al_(z))₂O₃ (0<x≦1, 0≦y<1,0≦z<1, x+y+z=1).

Advantageous Effect of the Invention

According to the invention, the production steps of the semiconductorelement can be simplified and the production costs thereof can bereduced.

In the invention, the undoped β-Ga₂O₃-based single crystal can be formedwith a high resistivity by the generally known crystal growth methods,e.g., HYPE (halide vapor phase epitaxy) and MBE (molecular beam epitaxy)(see [0042] described later). The semiconductor element can beconfigured by using for the element isolation the high-resistivityundoped β-Ga₂O₃-based single crystal and the low-concentration acceptorincluding β-Ga₂O₃-based single crystal doped a trace amount of acceptorimpurity thereinto.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic plan view showing a typical Ga₂O₃ MESFET in afirst embodiment of the present invention.

FIG. 1B is a schematic cross sectional view taken along a line I-I inFIG. 1A and viewed in an arrow direction.

FIG. 2 is a schematic cross sectional view taken along a line II-II inFIG. 1A and viewed in an arrow direction.

FIG. 3A is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MESFET in the first embodiment.

FIG. 3B is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MESFET in the first embodiment.

FIG. 3C is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MESFET in the first embodiment.

FIG. 3D is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MESFET in the first embodiment.

FIG. 3E is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MESFET in the first embodiment.

FIG. 4A is a schematic plan view showing a Ga₂O₃ MOSFET in a secondembodiment of the invention.

FIG. 4B is a schematic cross sectional view taken along a line IV-IV inFIG. 4A and viewed in an arrow direction.

FIG. 5 is a schematic cross sectional view taken along a line V-V inFIG. 4A and viewed in an arrow direction.

FIG. 6A is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6B is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6C is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6D is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6E is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6F is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6G is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 6H is a schematic explanatory cross sectional view showing a stepof manufacturing the Ga₂O₃ MOSFET in the second embodiment.

FIG. 7 is a schematic cross sectional view showing a semiconductordevice in Example.

FIG. 8 is a graph showing current-voltage characteristics betweenchannel layers in the semiconductor device in Example.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the invention will be specifically describedbelow in conjunction with the appended drawings.

First Embodiment

(General Configuration of Ga₂O₃ Semiconductor Element)

FIGS. 1A to 2 show a Ga₂O₃-based MESFET (Metal Semiconductor FieldEffect Transistor) 10 (hereinafter, simply referred to as “MESFET 10”)as a Ga₂O₃-based semiconductor element in the first embodiment.

The MESFET 10 has an undoped or low-concentration acceptorimpurity-including β-Ga₂O₃ single crystal layer (hereinafter, sometimessimply referred to as “β-Ga₂O₃ single crystal layer”) 12 formed on ahigh-resistivity substrate 11, a channel layer 13 formed in a channelregion of the β-Ga₂O₃ single crystal layer 12, and a source region 14and a drain region 15 formed in predetermined regions of the β-Ga₂O₃single crystal layer 12 and the channel layer 13.

The MESFET 10 also has a source electrode 16 formed on the source region14, a drain electrode 17 formed on the drain region 15, and a gateelectrode 18 formed on the channel layer 13 so as to be located betweenthe source electrode 16 and the drain electrode 17. The β-Ga₂O₃ singlecrystal layer 12 here is a high-resistivity layer which is undoped orcontains a low concentration of acceptor impurity.

(Configuration of High-Resistivity Substrate)

The high-resistivity substrate 11 is a substrate formed of aβ-Ga₂O₃-based single crystal doped with an acceptor impurity, e.g., Fe,Be, Mg or Zn, etc., and has high-resistivity due to the doping of theacceptor impurity.

To form the high-resistivity substrate 11 doped with, e.g., Fe as anacceptor impurity, for example, a Fe-doped high-resistivity β-Ga₂O₃single crystal is grown by, e.g., the EFG (Edge-defined Film-fed Growth)method and is then sliced and polished to a desired thickness.

The principal surface of the high-resistivity substrate 11 ispreferably, e.g., a surface rotated not less than 50° and not more than90° from the (100) plane of the β-Ga₂O₃ single crystal. In other words,an angle θ (0<θ≦90° formed between the principal surface of thehigh-resistivity substrate 11 and the (100) plane is preferably not lessthan 50°. Examples of the surface rotated not less than 50° and not morethan 90° from the (100) plane include a (010) plane, a (001) plane, a(−201) plane, a (101) plane and a (310) plane.

When the principal surface of the high-resistivity substrate 11 is asurface rotated not less than 50° and not more than 90° from the (100)plane, it is possible to effectively suppress re-evaporation of rawmaterials of the β-Ga₂O₃ crystal from the high-resistivity substrate 11at the time of epitaxially growing the β-Ga₂O₃ crystal on thehigh-resistivity substrate 11.

In detail, where a percentage of the re-evaporated raw material duringgrowth of the β-Ga₂O₃ crystal at a growth temperature of 500° C. isdefined as 0%, the percentage of the re-evaporated raw material can besuppressed to not more than 40% when the principal surface of thehigh-resistivity substrate 11 is a surface rotated not less than 50° andnot more than 90° from the (100) plane. It is thus possible to use notless than 60% of the supplied raw material to form the β-Ga₂O₃ crystal,which is preferable from the viewpoint of growth rate and manufacturingcost of the β-Ga₂O₃ crystal.

In the β-Ga₂O₃ crystal, the (100) plane comes to coincide with the (310)plane when rotated by 52.5° about the c-axis and comes to coincide withthe (010) plane when rotated by 90°. Meanwhile, the (100) plane comes tocoincide with the (101) plane when rotated by 53.8° about the b-axis,comes to coincide with the (001) plane when rotated by 76.3° and comesto coincide with the (−201) plane when rotated by 53.8°.

The principal surface of the high-resistivity substrate 11 is, e.g., the(010) plane, or a surface rotated within an angle range of not more than37.5° with respect to the (010) plane. In this case, a steep interfaceis obtained between the β-Ga₂O₃ single crystal layer 12 and the channellayer 13 since the surface of the β-Ga₂O₃ single crystal layer 12 can beflattened at the atomic level and it is more effective to preventleakage. It is possible to prevent uneven element uptake by the β-Ga₂O₃single crystal layer 12 and thereby to obtain the homogeneous β-Ga₂O₃single crystal layer 12. Note that, the (010) plane comes to coincidewith the (310) plane when rotated by 37.5° about the c-axis.

When (001) is the plane orientation of the principal surface of thehigh-resistivity substrate 11, the epitaxial growth rate of the β-Ga₂O₃single crystal on the high-resistivity substrate 11 is particularly highamong those plane orientations and it is possible to suppress diffusionof the acceptor impurity from the high-resistivity substrate 11 into theβ-Ga₂O₃ single crystal layer 12 and the channel layer 13. Thus, theplane orientation of the principal surface of the high-resistivitysubstrate 11 is preferably (001).

(Configurations of Undoped or Low-Concentration AcceptorImpurity-Including β-Ga₂O₃ single crystal layer)

The undoped or low-concentration acceptor impurity-including β-Ga₂O₃single crystal layer 12 is formed by epitaxially growing a β-Ga₂O₃single crystal on the high-resistivity substrate 11 as a base substrateand can serve as an element isolation region which electrically isolatesplural MESFETs from each other. A β-Ga₂O₃ single crystal formed by thisepitaxial growth has an element isolation region which does not containintentionally doped donor and acceptor impurities but contains theacceptor impurity diffused from the high-resistivity substrate 11 at aconcentration of less than 1×10¹⁶ cm⁻³.

In the first embodiment, the undoped β-Ga₂O₃ single crystal layer 12 tobe such element isolation region is a region which contains anunintentional donor and/or acceptor impurity at a concentration of lessthan 1×10¹⁵ cm⁻³. A trace amount of acceptor impurity, e.g., about lessthan 1×10¹⁶ cm⁻³ can be doped into this region to form alow-concentration acceptor impurity-including region. This can enhancethe safety against the contamination of an unintentional donor impurity.

The β-Ga₂O₃ single crystal layer 12 can be formed by epitaxial growthusing, e.g., the MBE method. The thickness of the β-Ga₂O₃ single crystallayer 12 is, e.g., about 10 to 10000 nm. When a Ga metal with 99.9999%purity commercially available from Koj undo Chemical Laboratory Co.,Ltd. and a mixed gas of 95% oxygen and 5% ozone produced by an ozonegenerator are used as raw materials in this case, it is possible toobtain the undoped β-Ga₂O₃ single crystal layer 12 with a donorconcentration of less than 1×10¹⁵ cm⁻³.

To estimate resistivity of the β-Ga₂O₃ single crystal layer 12, a 3μm-thick undoped β-Ga₂O₃ single crystal layer was formed on a 600μm-thick n⁺ substrate and current-voltage characteristics were measured.The n⁺ substrate was doped with Sn at a concentration of about 10¹⁸ cm⁻³and had a resistivity of about 0.01 Ωcm. In this measurement, a circularPt/Ti/Au electrode having a diameter of 200 μm was formed on the β-Ga₂O₃single crystal layer and also a Ti/Au electrode in ohmic contact withthe n⁺ substrate was formed on the entire lower surface of the n⁺substrate. A resistance value was calculated based on the result ofcurrent-voltage measurement conducted by applying voltage between theelectrodes, and resistivity of the β-Ga₂O₃ single crystal layer wasfurther calculated based on the thickness of the β-Ga₂O₃ single crystallayer, the electrode area and the obtained resistance value. Thecalculated resistivity of the β-Ga₂O₃ single crystal layer was about2.5×10⁷ Ωcm. In this regard, resistivity is about the same even when theβ-Ga₂O₃ single crystal layer contains a trace amount of acceptorimpurity at a concentration of about less than 1×10¹⁶ cm⁻³.

Alternatively, a β-Ga₂O₃-based single crystal layer, which is formed ofa β-Ga₂O₃-based single crystal other than the β-Ga₂O₃ single crystal andis undoped or contains an acceptor impurity at a concentration of lessthan 1×10¹⁶ cm⁻³, may be used in place of the β-Ga₂O₃ single crystallayer 12. Resistivity of general β-Ga₂O₃-based single crystal layers issubstantially the same as resistivity of the β-Ga₂O₃ single crystallayer.

(Configuration of the Channel Layer)

The channel layer 13 is an n-type layer formed of a β-Ga₂O₃-based singlecrystal containing a donor impurity. The donor impurity is, e.g., aGroup IV element such as Si or Sn. The faces of the channel layer 13except the top surface are surrounded by the undoped orlow-concentration acceptor impurity-including region of the β-Ga₂O₃single crystal layer 12. Meanwhile, a donor impurity is doped into thechannel layer 13 by ion implantation or thermal diffusion.

(Configuration of Source Region and Drain Region)

The source region 14 and the drain region 15 are formed by doping adonor impurity, e.g., Si or Sn, etc., into the β-Ga₂O₃ single crystallayer 12. The doping is performed by ion implantation or thermaldiffusion. The donor impurity contained in the source region 14 and thedrain region 15 may be the same as or different from the donor impuritycontained in the channel layer 13.

The thickness of the source region 14 and the drain region 15 is, e.g.,about 150 nm. In the illustrated example, the donor impurityconcentration of the source region 14 and the drain region 15 is, e.g.,about 5×10¹⁹ cm⁻³ and is higher than the donor impurity concentration ofthe channel layer 13.

(Configuration of the Electrodes)

The source region 14 and the drain region 15 are respectivelyelectrically connected to the source electrode 16 and the drainelectrode 17. The source electrode 16, the drain electrode 17 and thegate electrode 18 are formed of, e.g., a metal such as Au, Al, Ti, Sn,Ge, In, Ni, Co, Pt, W, Mo, Cr, Cu or Pb, an alloy containing two or moreof such metals, or a conductive compound such as ITO.

The source electrode 16, the drain electrode 17 and the gate electrode18 may have a laminated structure consisting of two or more layersformed of two different metals, e.g., Ti/Al, Ti/Au, Pt/Ti/Au, Al/Au,Ni/Au or Au/Ni.

(Operation of Ga₂O₃ Semiconductor Element)

The MESFET 10 configured as such can be a normally-on type or anormally-off type depending on the donor concentration and the thicknessof the channel layer 13 directly below the gate electrode 18.

In case that the MESFET 10 is a normally-on type, the source electrode16 is electrically connected to the drain electrode 17 via the channellayer 13. Therefore, if a voltage is applied between the sourceelectrode 16 and the drain electrode 17 in a state that a voltage is notapplied to the gate electrode 18, a current passes through from thesource electrode 16 to the drain electrode 17.

On the other hand, if a voltage is applied to the gate electrode 18, adepletion layer is formed in the channel layer 13 in a region under thegate electrode 18. A current does not pass through from the sourceelectrode 16 to the drain electrode 17 even if a voltage is appliedbetween the source electrode 16 and the drain electrode 17.

In case that the MESFET 10 is a normally-off type, a current does notpass through in a state that a voltage is not applied to the gateelectrode 18 even if a voltage is applied between the source electrode16 and the drain electrode 17.

On the other hand, if a voltage is applied to the gate electrode 18, thedepletion layer in the channel layer 13 in the region under the gateelectrode 18 is narrowed. A current passes through from the sourceelectrode 16 to the drain electrode 17 if a voltage is applied betweenthe source electrode 16 and the drain electrode 17.

(Production Method for the Ga₂O₃ Semiconductor Element)

Next, a production method for the MESFET 10 configured as describedabove will be described in reference to FIGS. 3A to 3E.

The production method for the MESFET 10 includes the followingsequential steps: a step of forming the high-resistivity substrate 11, astep of forming the β-Ga₂O₃ single crystal layer 12 on thehigh-resistivity substrate 11, a step of forming the channel layer 13 inthe β-Ga₂O₃ single crystal layer 12, a step of forming the source region14 and the drain region 15 spanning from the channel layer 13 to theβ-Ga₂O₃ single crystal layer 12, and a step of forming the sourceelectrode 16 on the source region 14, the drain electrode 17 on thedrain region 15 and the gate electrode 18 on the channel layer 13between the source electrode 16 and the drain electrode 17.

(Step of Forming the High-Resistivity Substrate)

To produce a Ga₂O₃-based semiconductor element, firstly, thehigh-resistivity substrate 11 is formed by slicing and polishing, e.g.,a Fe-doped high-resistivity β-Ga₂O₃ single crystal grown by the EFGmethod into a desired thickness, as shown in FIG. 3A. The principalsurface of the high-resistivity substrate 11 is, e.g., the (010) plane.

(Step of Forming the β-Ga₂O₃ Single Crystal Layer)

To form the β-Ga₂O₃ single crystal layer 12, a β-Ga₂O₃ single crystal isepitaxially grown on the high-resistivity substrate 11 as a basesubstrate by, e.g., the HYPE method or the molecular beam epitaxymethod, as shown in FIG. 3B. To obtain the undopedβ-Ga₂O₃ single crystallayer 12, the thickness of the β-Ga₂O₃ single crystal layer 12 isadjusted to, e.g., about 10 to 10000 nm.

A β-Ga₂O₃-based single crystal having an undoped region with a donorand/or acceptor concentration of less than 1×10¹⁵ cm⁻³ is formed by thisepitaxial growth. The undoped region, if required, is doped with a traceamount of acceptor impurity at a concentration of, e.g., about 1×10¹⁶cm⁻³.

(Step of Forming the Channel Layer)

The method of introducing the donor impurity into the β-Ga₂O₃ singlecrystal layer 12 is, e.g., ion implantation. The ion implantation methodis used in this step and as shown in FIG. 3C, the channel layer 13 isformed in the β-Ga₂O₃ single crystal layer 12 by implanting ions of ann-type dopant such as Si into the β-Ga₂O₃ single crystal layer 12 inmultiple stages.

To obtain a normally-on Ga₂O₃-based MESFET, the n-type dopant isimplanted to a depth of 300 nm so that the average n-type dopantconcentration is 3×10¹⁷ cm⁻³. Meanwhile, to obtain a normally-offGa₂O₃-based MESFET, the n-type dopant is implanted to a depth of 300 nmso that the average n-type dopant concentration is 1×10¹⁶ cm⁻³.

(Step of Forming the Source Region and Drain Region)

In FIG. 3D, the source region 14 and the drain region 15 are formed by,e.g., the ion implantation method, etc. An n-type dopant such as Si orSn is implanted by multistage ion implantation into the channel layer 13or into a portion spanning between the channel layer 13 and the β-Ga₂O₃single crystal layer 12. To obtain the source region 14 and the drainregion 15 which have a higher concentration than the channel layer 13,the n-type dopant is implanted to a depth of 150 nm so that the averagen-type dopant concentration is 5×10¹⁹ cm⁻³.

The n-type dopant is implanted in multiple stages into the donorimpurity-doped region of the channel layer 13 using, e.g., a mask formedby photolithography. After implanting the n-type dopant in multiplestages, the n-type dopants implanted into the channel layer 13, thesource region 14 and the drain region 15 are activated by activationannealing in a nitrogen atmosphere at 950° C. for 30 minutes.

(Step of Forming the Electrodes)

In FIG. 3E, the source electrode 16 is formed on the source region 14and the drain electrode 17 is formed on the drain region 15. The gateelectrode 18 is formed on the channel layer 13 so as to be locatedbetween the source electrode 16 and the drain electrode 17.

The source electrode and the drain electrode are formed as follows: amask pattern is formed on the upper surfaces of the β-Ga₂O₃ singlecrystal layer 12, the channel layer 13, the source region 14 and thedrain region 15 by, e.g., photolithography, a metal film such as Ti/Auis subsequently deposited on the β-Ga₂O₃ single crystal layer 12, thechannel layer 13, the source region 14, the drain region 15 and theentire surface of the mask pattern, and the mask pattern and the metalfilm except a portion in the openings of the mask pattern are removed bylift-off. The source electrode 16 and the drain electrode 17 are therebyformed.

After forming the source electrode 16 and the drain electrode 17, theelectrodes are annealed in, e.g., a nitrogen atmosphere at 450° C. for 1minute. Contact resistance between the source region 14 and the sourceelectrode 16 and between the drain region 15 and the drain electrode 17can be reduced by annealing the electrodes.

The gate electrode is formed as follows: a mask pattern is formed on theupper surfaces of the β-Ga₂O₃ single crystal layer 12, the channel layer13, the source region 14, the drain region 15, the source electrode 16and the drain electrode 17 by, e.g., photolithography, a metal film suchas Pt/Ti/Au is subsequently deposited on the entire surface, and themask pattern and the metal film except a portion in the opening of themask pattern are removed by lift-off. The gate electrode 18 is therebyformed. The whole process is completed with this step.

(Effects of the First Embodiment)

The MESFET 10 configured as described above and the production methodthereof in the first embodiment have the following effects in additionto the effects described above.

(1) It is possible to obtain the MESFET 10 having an element isolationstructure which is not formed by element isolation technique usingacceptor impurity ion implantation or a mesa process.

(2) The production time can be shorter than when produced by a methodusing acceptor impurity ion implantation or a mesa process and it isalso possible to produce the MESFET 10 at low cost.

(3) Since the acceptor impurity diffused from the high-resistivitysubstrate 11 is barely contained in the channel layer 13, an increase inresistance of the channel layer 13 due to carrier compensation can besuppressed.

Second Embodiment

FIGS. 4A to 5 show a Ga₂O₃-based MOSFET (Metal Oxide Semiconductor FieldEffect Transistor) 20 (hereinafter, simply referred to as “MOSFET 20”)as a Ga₂O₃ semiconductor element in the second embodiment. In thesedrawings, members substantially the same as those in the firstembodiment are denoted by the same names and the same referencenumerals. Therefore, the detailed description of such members will beomitted.

The second embodiment is different from the first embodiment in that theGa₂O₃ semiconductor element is a MOSFET.

(Configuration of the Ga₂O₃ Semiconductor Element)

In FIGS. 4A and 4B, the surface of the β-Ga₂O₃ single crystal layer 12is covered with a gate insulating film 19. The gate insulating film 19is formed of, e.g., an insulating material such as silicon oxide (SiO₂)or sapphire (Al₂O₃). The thickness of the gate insulating film 19 is,e.g., about 20 nm.

The source electrode 16 and the drain electrode 17 are partially exposedon the surface, as shown in FIGS. 4A and 5. Meanwhile, the gateelectrode 18 is formed on the channel layer 13 via the gate insulatingfilm 19 so as to be located between the source electrode 16 and thedrain electrode 17.

(Production Method for the Ga₂O₃ Semiconductor Element)

As shown in FIGS. 6A to 6H, the production method for the MOSFET 20includes the following sequential steps: a step of forming thehigh-resistivity substrate 11, a step of forming the β-Ga₂O₃ singlecrystal layer 12, a step of forming the channel layer 13, a step offorming the source region 14 and the drain region 15, a step of formingthe source electrode 16 and the drain electrode 17, a step of formingthe gate insulating film 19, a step of forming the gate electrode 18 anda step of etching a portion of the gate insulating film 19.

The continuous process from the step of forming the β-Ga₂O₃ singlecrystal layer 12 to the step of forming the source electrode 16 and thedrain electrode 17 is performed in the same manner as in the firstembodiment. Since the continuous process from the step of forming theβ-Ga₂O₃ single crystal layer 12 to the step of forming the sourceelectrode 16 and the drain electrode 17 is illustrated in FIGS. 6A to6E, the detailed explanation of the process of forming thereof will beomitted.

The second embodiment is different from the first embodiment in that asshown in FIGS. 6F to 6H, the step of forming the gate insulating film19, the step of forming the gate electrode 18 and the step of etching aportion of the gate insulating film 19 are performed after the step offorming the source electrode 16 and the drain electrode 17.

(Step of Forming the Gate Insulating Film)

In FIG. 6F, a material consisting mainly of an oxide insulation such asAl₂O₃ is deposited on the entire surface on/above the β-Ga₂O₃ singlecrystal layer 12 to form the gate insulating film 19. The gateinsulating film 19 is formed by, e.g., the ALD (Atomic Layer Deposition)method using an oxidant such as oxygen plasma. Alternatively, anothermethod such as CVD method or PVD (Physical Vapor Deposition) method maybe used to form the gate insulating film 19 instead of using the ALDmethod.

(Step of Forming the Gate Electrode)

As shown in FIG. 6G, the gate electrode 18 is formed on the gateinsulating film 19 so as to be located between the source electrode 16and the drain electrode 17. To form the gate electrode 18, a maskpattern is formed on the gate insulating film 19 by, e.g.,photolithography, a metal film such as Pt/Ti/Au is subsequentlydeposited on the gate insulating film 19 and the mask pattern, and themask pattern and the metal film are removed by lift-off.

(Step of Etching the Gate Insulating Film)

After forming the gate electrode 18 in FIG. 6G, the gate insulating film19 on the source electrode 16 and the drain electrode 17 is removed bydry etching, etc., so that the source electrode 16 and the drainelectrode 17 are partially exposed on the surface. The whole process iscompleted with this step.

(Effects of the Second Embodiment)

The same effects as those in the first embodiment are obtained also inthe second embodiment.

Example

In this Example, two MOSFETs 20 of the second embodiment were formedside by side on the same substrate and the function of the undopedβ-Ga₂O₃ single crystal layer 12 as an element isolation region wasevaluated. The evaluation of the function of the element isolationregion was conducted in the middle of formation of the MOSFET 20 (in thestate shown in FIG. 6E).

(Configuration of the Semiconductor Device)

FIG. 7 is a schematic cross sectional view showing a semiconductordevice 30 having two MOSFETs 20 (hereinafter, referred as MOSFETs 20 aand 20 b). In the semiconductor device 30, a distance D between thechannel layer 13 of the MOSFET 20 a and the channel layer 13 of theMOSFET 20 b is 10 μm. In the MOSFETs 20 a and 20 b, the source region 14and the drain region 15 in the channel layer have the fixed width in adirection perpendicular to the plane of FIG. 7 (the width in a verticaldirection of FIG. 4A) which is 100 μm. This width is about several μmsmaller than the width of the channel layer 13, such that the sourceregion 14 and the drain region 15 are located on the inner side of thechannel layer 13. Meanwhile, the thickness T of the β-Ga₂O₃ singlecrystal layer 12 is 0.5, 1.0 or 1.5 μm.

(Production Method for the Semiconductor Device)

Firstly, a Fe-doped high-resistivity β-Ga₂O₃ single crystal was grown bythe EFG method. The crystal was sliced to a thickness of 1 mm so as tohave the (010) plane as a principal surface, then ground/polished andlastly cleaned with organic solvent and acid, thereby making thehigh-resistivity substrate 11 having a thickness of 0.65 mm

Next, the undoped β-Ga₂O₃ single crystal layer 12 was formed on thehigh-resistivity substrate 11 by the MBE method. A Ga metal with99.99999% purity and a mixed gas of 95% oxygen and 5% ozone produced byan ozone generator were used as raw materials of the β-Ga₂O₃ singlecrystal layer 12. The β-Ga₂O₃ single crystal layer 12 was grown at atemperature of 560° C. so as to have a film thickness of 0.5, 1.0 or 1.5μm.

Next, ion implantation was performed to form the channel layers 13 ofthe MOSFETs 20 a and 20 b. Si was selected as a donor impurity. Aphotoresist and an implantation mask made of SiO₂ were formed on theβ-Ga₂O₃ single crystal layer 12 by photolithography so as to haveopenings in regions for forming the channel layers 13 and Si wassubsequently implanted, thereby forming the channel layers 13 with a boxprofile having a Si concentration of 3×10¹⁷ cm⁻³ and a depth of 300 nm.After the implantation, the implantation mask and the photoresistthereon were removed by organic cleaning, O₂ ashing and buffered HFcleaning.

Next, ion implantation was performed to form the source regions 14 andthe drain regions 15 of the MOSFETs 20 a and 20 b. An implantation maskmade of SiO₂ was formed by photolithography and Si was subsequentlyimplanted, thereby forming the source regions 14 and the drain regions15 with a box profile having a Si concentration of 5×10¹⁹ cm⁻³ and adepth of 150 nm. After the implantation, the implantation mask and thephotoresist thereon were removed by organic cleaning, O₂ ashing andbuffered HF cleaning.

Next, the ion-implanted donor impurity was activated by annealing in anitrogen atmosphere at 950° C. for 30 minutes.

Next, using the lift-off method, the source electrodes 16 and the drainelectrodes 17 of the MOSFETs 20 a and 20 b were formed so as to have aTi/Au two-layer structure. After forming the source electrodes 16 andthe drain electrodes 17, annealing was performed in a nitrogenatmosphere at 450° C. for 1 minute to reduce contact resistance betweenthe source electrode 16 and source region 14 and between the drainelectrode 17 and the drain region 15 and to obtain good ohmic contact.

(Evaluation of Element Isolation Performance)

Current-voltage characteristics between the channel layer 13 of theMOSFET 20 a and the channel layer 13 of the MOSFET 20 b were measured byKEITHLEY 4200-SCS Semiconductor Parameter Analyzer and MX-1100 seriesProber manufactured by Vector Semiconductor Co., Ltd. In thismeasurement, probes of the prober were placed on the drain electrode 17of the MOSFET 20 a and the source electrode 16 of the MOSFET 20 b.

FIG. 8 is a graph showing current-voltage characteristics measuredbetween the channel layer 13 of the MOSFET 20 a and the channel layer 13of the MOSFET 20 b. FIG. 8 includes data measured at three differentmeasurement points on each of three samples respectively having the 0.5,1.0 or 1.5 μm-thick β-Ga₂O₃ single crystal layers 12.

Resistivity of the undoped β-Ga₂O₃ single crystal layer 12 was estimatedbased on the resistance value calculated from the inclination of line inFIG. 8 and the size of the undoped β-Ga₂O₃ single crystal layer 12between the channel layers. As a result, resistivity of the β-Ga₂O₃single crystal layer 12 was about 2 to 3×10¹⁰ Ωcm when having thethickness T of 0.5 μm, 1 to 2×10¹⁰ Ωcm when having the thickness T of1.0 μm and 2 to 3×10¹⁰ Ωcm when having the thickness T of 1.5 μm. Sincethe estimated resistivity does not depend on the thickness of theundoped β-Ga₂O₃ single crystal layer 12, the measured current isconsidered to be a leakage current passing through the surface, etc., ofthe film, not a current passing inside the undoped β-Ga₂O₃ singlecrystal layer 12. Based on this, it is presumed that the actualresistivity of the undoped β-Ga₂O₃ single crystal layer 12 is higherthan the numerical values mentioned above.

It was found from this evaluation that the undoped β-Ga₂O₃ singlecrystal layer 12 between the channel layer 13 of the MOSFET 20 a and thechannel layer 13 of the MOSFET 20 b functions as an element isolationregion having very high insulating properties.

Also when the function of the undoped β-Ga₂O₃ single crystal layer 12 ofthe MESFET 10 in the first embodiment was evaluated by the same method,the similar result was obtained, in which the undoped β-Ga₂O₃ singlecrystal layer 12 has sufficient resistivity and functions as an elementisolation region having very high insulating properties.

Although the typical embodiments, Example, modifications and illustratedexamples of the invention have been described, the invention accordingto claims is not intended to be limited to the embodiments, Example,modifications and illustrated examples, as obvious from the abovedescription. Therefore, it should be noted that all combinations of thefeatures described in the embodiments, modifications and illustratedexamples are not necessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

Provided are a semiconductor element and a production method thereofwhich allow production steps to be simplified and production costs to bereduced.

REFERENCE SIGNS LIST

-   10: Ga₂O₃ MESFET-   11: HIGH-RESISTIVITY SUBSTRATE-   12: β-Ga₂O₃ SINGLE CRYSTAL LAYER-   13: CHANNEL LAYER-   14: SOURCE REGION-   15: DRAIN REGION-   16: SOURCE ELECTRODE-   17: DRAIN ELECTRODE-   18: GATE ELECTRODE-   19: GATE INSULATING FILM-   20: Ga₂O₃ MOSFET

1. A semiconductor element, comprising: a high-resistivity substratethat comprises a β-Ga₂O₃-based single crystal including an acceptorimpurity; an undoped β-Ga₂O₃-based single crystal layer formed on thehigh-resistivity substrate; and an n-type channel layer that comprises aside surface surrounded by the undoped β-Ga₂O₃-based single crystallayer, wherein the undoped β-Ga₂O₃-based single crystal layer comprisesan element isolation region.
 2. A semiconductor element, comprising: ahigh-resistivity substrate that comprises a β-Ga₂O₃-based single crystalcomprising an acceptor impurity; an undoped β-Ga₂O₃-based single crystallayer formed on the high-resistivity substrate; and an n-type channellayer that comprises a side surface and a bottom surface on a side ofthe substrate that are surrounded by the undoped β-Ga₂O₃-based singlecrystal layer, wherein the undoped β-Ga₂O₃-based single crystal layercomprises an element isolation region.
 3. The semiconductor elementaccording to claim 1, wherein the undoped β-Ga₂O₃-based single crystallayer is a region that includes an unintentional donor and/or acceptorimpurity at a concentration of less than 1×10¹⁵ cm⁻³.
 4. Thesemiconductor element according to claim 1, wherein a concentration of adonor impurity doped into the n-type channel layer is set to be higherthan a concentration of an acceptor impurity of the undopedβ-Ga₂O₃-based single crystal layer.
 5. The semiconductor elementaccording to claim 1, comprising a MESFET or MOSFET.
 6. Thesemiconductor element according to claim 1, further comprising anundoped region between an n-type channel region and an n-type channelregion.
 7. The semiconductor element according to claim 1, wherein theundoped β-Ga₂O₃-based single crystal layer is located between thehigh-resistivity substrate and the n-type channel layer.
 8. Asemiconductor element, comprising: a high-resistivity substrate thatcomprises a β-Ga₂O₃-based single crystal including an acceptor impurity;a low-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer formed on the high-resistivity substrate; and an n-typechannel layer that comprises a side surface and a bottom surface on aside of the substrate that are surrounded by the low-concentrationacceptor impurity-including β-Ga₂O₃-based single crystal layer, whereinthe low-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises an element isolation region.
 9. Thesemiconductor element according to claim 8, wherein thelow-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises a region that includes the acceptor impuritydiffused from the high-resistivity substrate at a concentration of lessthan 1×10¹⁶ cm⁻³.
 10. The semiconductor element according to claim 8,wherein a donor concentration of the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer is set to be lowerthan a concentration of the acceptor impurity diffused from thehigh-resistivity substrate, and wherein a concentration of a donorimpurity doped into the n-type channel layer is higher than aconcentration of an acceptor impurity of the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer.
 11. Thesemiconductor element according to claim 8, wherein thelow-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises a region that includes an intentionally dopedacceptor impurity at a concentration of less than 1×10¹⁶ cm⁻³.
 12. Thesemiconductor element according to claim 8, wherein the n-type channellayer comprises a side surface and a bottom surface on a side of thesubstrate that are surrounded by the acceptor impurity-includingβ-Ga₂O₃-based single crystal layer of a same element and a sameconcentration.
 13. A production method for a semiconductor element,comprising: forming an undoped β-Ga₂O₃-based single crystal layer on ahigh-resistivity substrate that comprises aβ-Ga₂O₃-based single crystalincluding an acceptor impurity; and forming an n-type channel layer bydoping a donor impurity into a predetermined region of the undopedβ-Ga₂O₃-based single crystal layer such that a side surface thereof issurrounded by the undoped β-Ga₂O₃-based single crystal layer, whereinthe undoped β-Ga₂O₃-based single crystal layer comprises an elementisolation region.
 14. A production method for a semiconductor element,comprising: forming a low-concentration acceptor impurity-includingβ-Ga₂O₃-based single crystal layer on a high-resistivity substrate thatcomprises a β-Ga₂O₃-based single crystal including an acceptor impurity;and forming an n-type channel layer by doping a donor impurity into apredetermined region of the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer such that a sidesurface and a bottom surface on a side of the substrate are surroundedby the low-concentration acceptor impurity-including β-Ga₂O₃-basedsingle crystal layer, wherein the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer comprises anelement isolation region.
 15. The production method for a semiconductorelement according to claim 14, wherein the forming the low-concentrationacceptor impurity-including a β-Ga₂O₃-based single crystal layercomprises doping an acceptor impurity of less than 1×10¹⁶ cm⁻³ into anundoped β-Ga₂O₃-based single crystal layer to form the low-concentrationacceptor impurity-including β-Ga₂O₃-based single crystal layer.
 16. Thesemiconductor element according to claim 2, wherein the undopedβ-Ga₂O₃-based single crystal layer is a region that includes anunintentional donor and/or acceptor impurity at a concentration of lessthan 1×10¹⁵ cm⁻³.
 17. The semiconductor element according to claim 2,wherein a concentration of a donor impurity doped into the n-typechannel layer is set to be higher than a concentration of an acceptorimpurity of the undoped β-Ga₂O₃-based single crystal layer.
 18. Thesemiconductor element according to claim 2, comprising a MESFET orMOSFET.
 19. The semiconductor element according to claim 2, furthercomprising an undoped region between an n-type channel region and ann-type channel region.
 20. The semiconductor element according to claim2, wherein the undoped β-Ga₂O₃-based single crystal layer is locatedbetween the high-resistivity substrate and the n-type channel layer. 21.The semiconductor element according to claim 9, wherein a donorconcentration of the low-concentration acceptor impurity-includingβ-Ga₂O₃-based single crystal layer is set to be lower than aconcentration of the acceptor impurity diffused from thehigh-resistivity substrate, and wherein a concentration of a donorimpurity doped into the n-type channel layer is higher than aconcentration of an acceptor impurity of the low-concentration acceptorimpurity-including β-Ga₂O₃-based single crystal layer.
 22. Thesemiconductor element according to claim 9, wherein thelow-concentration acceptor impurity-including β-Ga₂O₃-based singlecrystal layer comprises a region that includes an intentionally dopedacceptor impurity at a concentration of less than 1×10¹⁶ cm⁻³.